In the treatment of sulfuric acid-containing waste waters formed, for example, in metallurgical processes, an aqueous ammonium alum solution is sometimes formed, which contains iron(III) ions. Chemically pure ammonium alum has the formula NH.sub.4 Al(SO.sub.4).sub.2, and the compound sometimes includes 12 molecules of water of crystallization. When ammonium alum is crystallized from a solution which contains Fe.sup.3+ ions, the trivalent iron because of its affinity to aluminum, is included in the crystalline structure in a relatively high concentration.